In the field of packaging, both shrinkable and non-shrinkable multilayer films designed to hold goods must often fulfill multiple requirements.
For example, when packaging goods that have sharp edges or needle-like protrusions, it is important that the multilayer films used in the packaging of such goods have excellent puncture resistance.
A solution to such puncture resistance requirements can be provided by a multilayer film comprising a layer containing polyamide, or blends thereof. For example, EP1296830 describes a multilayer film comprising an intermediate layer of polyamide having good puncture resistance properties, suitable for packaging deep-frozen foods such as ribs and seafood such as crabs.
On the other hand, such multilayer films must also display other properties such as the tendency to shrink under the influence of heat and adopt the form of the packaged good, a property known as “heat shrink”.
In most packaging applications such as shrink bags used for food and in particular for meat packaging, a “heat shrink” of at least 40% is required, but the inclusion of a polyamide layer to enhance puncture resistance generally also reduces the tendency of a multilayer film to shrink under heat, and so a compromise between these two desirable properties must be struck.
Ionomers are known for their excellent ability to shrink under heat, and depending on the degree of neutralization of the ionomer, the “heat shrink” of a multilayer film comprising a layer of ionomer can reach 50% and even higher values.
However, multilayer films comprising a layer of ionomer have only moderate puncture resistance compared to similar multilayer structures that contain stiffer components such as polyamide and are therefore not used extensively to package sharp or pointy edges such as meat cuts having salient bone fragments or splinters.
It is believed that blends of polyamide and ionomer would result in a polymeric material combining the puncture resistance of polyamide and to heat shrink performance of ionomers, but to a strongly diminished extent, making such compositions less desirable.
There exists the possibility to combine multiple layers in one film in order to produce a film combining the advantages of each of the individual layers, but since the economic cost of such a film increases with its complexity, this is not always advantageous. In many cases, multilayer films must be constructed by sequential lamination operations, which add expense. Furthermore, the inclusion of stiff polyamide layers in order to increase puncture resistance leads to complications from an environmental point of view, since the recycling of such structures is inherently problematic due to the chemical incompatibility between polyamides and polyolefins.
Therefore, there exists a need to provide a multilayer film that combines good puncture resistance and, more importantly high “heat shrink” properties in one layer, which can be manufactured at low economic cost, and which can be recycled more easily.
EP1423408 discloses sealant layers comprising blends of ionomers and cyclic olefin polymers having relatively good puncture resistance, but is silent on the heat shrink behavior of such blends in oriented multilayer films.